Referring to FIG. 1, there is shown a schematic diagram of a typical cellular mobile communication system 10. The system 10 includes a Core Network (CN) 12, a Radio Access Network (RAN) 14, and a plurality of Mobile Stations (MS) 16. The RAN 14 is divided into controller nodes 18 and Base Transceiver Station (BTS) nodes 20. Of course, as will be appreciated by those having ordinary skill in the art, the RAN 14 may be made up of several RANs, each having one or more controller nodes 18 and BTS nodes 20. The hierarchy of the system is such that the CN 12 is connected to several controller nodes 18, each controller node 18 is connected to several BTS nodes 20, and each BTS node 20 services one or more MS 16.
Due to error characteristics associated with the radio interface between an MS 16 and a servicing BTS node 20, an Automatic Repeat Request (ARQ) protocol can optionally be executed between the MS 16 and the RAN 14 to reduce the residual error rate. The function of the ARQ protocol is to take care of errors that are introduced as a result of the radio interface (e.g., due to interference). However, when the MS 16 moves around within the system 10, a handover may occur that results in moving the execution of the ARQ protocol between different controller nodes 18. To insure that no user data is lost during a handover, certain mechanisms must be implemented. There are presently three known mechanisms for securing user data in the case of a handover of the ARQ protocol between different controller nodes 18.
In the first known mechanism for securing user data in the case of a handover of the ARQ protocol between different controller nodes 18, which is adequately described by R. Cohen et al. in "Handover in a Micro-Cell Packet Switched Mobile Network", ACM Journal of Wireless Networks, Vol. 2, No. 1, 1996, pp. 13-25, and by E. Ayanoglu et al. in "AIRMAIL: A Link-Layer Protocol for Wireless Networks", ACM/Baltzer Wireless Networks Journal, Vol. 1, 1995, pp. 47-60, when the handover is performed, the entire protocol state, including the state variables and buffers, from the ARQ protocol entity in the RAN 14 are moved/transferred from an origination controller node 18 to a destination controller node 18. Using this mechanism, the ARQ protocol entity in the MS 16 does not need to know when the handover occurs. In the case of a General Packet Radio Service (GPRS) system having two or more Serving GPRS Support Nodes (SGSN's) wherein an inter-SGSN handover takes place, only the downlink buffer is moved/transferred from the origination SGSN to the destination SGSN, and the protocol states of the buffers are synchronized between the MS 16 and the destination SGSN by means of handover signaling (see GSM 03.60--"Service Description").
The main benefits of this first mechanism are that no unnecessary re-transmission of the user data is required over the radio interface, and that the ARQ protocol in the MS 16 can be unaware of the handover, which also makes the implementation less expensive. However, this first mechanism is limited to intra-system handovers, where the same ARQ protocol with the same configuration is used throughout the system. Thus, it will no longer be useful in future systems where it will be possible to use different ARQ protocol configurations within the same RAN, and where there can be different sizes of protocol data units (PDU's) associated with the different ARQ protocol configurations. In addition, it can be very complex to move an entire protocol state.
In the second known mechanism for securing user data in the case of a handover of the ARQ protocol between different controller nodes 18, which is specifically used in GPRS systems, the user data is secured by having 2 levels of ARQ protocols in the system 10. The first ARQ protocol, called a Radio Link Control (RLC) protocol, is executed between an MS 16 and the RAN 14 (e.g., at a Base Station Controller (BSC) node) and is used to take care of errors that are introduced as a result of the radio interface (see GSM 04.60--"Radio Link Control/Medium Access Control"). The second ARQ protocol, called a Logical Link Control (LLC) protocol, is executed between an MS 16 and the CN 12 (e.g., at an SGSN node) (see GSM 04.64--"Logical Link Control (LLC) Layer Specification"). When a handover takes place, potentially lost user data is retransmitted by the ARQ protocol within the LLC protocol. The RLC protocol, on the other hand, is re-started in both the MS 16 and the BSC when a handover is performed.
The main benefit of this second mechanism is that it can handle inter-system handovers. However, this second mechanism has major disadvantages. For instance, unnecessary radio resources are wasted due to the overhead associated with the second ARQ protocol. In GPRS, the overhead that is transmitted with a third layer (L3) PDU is on the order of 7 bytes. This can be compared to the size of a Van Jacobsen compressed Transmission Control Protocol (TCP) acknowledgment, which is under 10 bytes when using a Point-to-Point Protocol (PPP). Thus, when transmitting TCP acknowledgments in an L3 PDU, the size is almost doubled. Another disadvantage of this second mechanism is that the cost in terms of memory and processing power of having 2 levels of ARQ protocols in the MS 16 is significantly higher than for a single ARQ protocol.
In the third known mechanism for securing user data in the case of a handover of the ARQ protocol between different controller nodes 18, a sender of second layer (L2) ARQ protocol PDUs is required to keep all the L2 PDUs, carrying an L3 PDU, in a buffer until the whole L3 PDU has been acknowledged. Then, when a handover is performed, all the L3 PDUs are moved to the new L2_ARQ protocol entity, which then segments these L3 PDUs into new L2 PDUs and retransmits them.
Similar to the second mechanism, the main benefit of this third mechanism is that it can handle inter-system handovers. However, this third mechanism also has major disadvantages. For instance, extra buffer space is required because the sender of the L2_ARQ protocol PDUs is required to keep all the L2 PDUs, carrying a L3 PDU, in a buffer until the whole L3 PDU has been acknowledged. Also, when a handover takes place, all L2 PDUs of an L3 PDU are retransmitted by the new L2_ARQ protocol. That is, even the L2 PDUs which were previously acknowledged are retransmitted. This is of course not optimal and a major disadvantage of this third mechanism.
In view of the foregoing, it would be desirable to provide a technique for providing a secure link between a mobile station and a core network during a handover or a protocol reconfiguration in a mobile communication system which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to provide a technique for providing a secure link between a mobile station and a core network during a handover or a protocol reconfiguration in a mobile communication system which does not transfer the entire state of an ARQ protocol, which does not use a second ARQ protocol level, which does not retransmit L2_ARQ PDUs which have already been acknowledged, and which does not need to store already acknowledged L2_ARQ PDUs in a buffer of the sending L2_ARQ protocol entity.